1. Field
This disclosure relates to stereoscopy.
2. Description of the Related Art
Humans view the environment three-dimensionally using binocular vision. Binocular vision is both a visual system and an analytical system. Our brain perceives both distance and speed based, in part, on triangulating visual light information received by the retinas of our respective laterally separated, forward facing eyes. Since both eyes are forward facing, the fields of view of each of our eyes overlap, with each eye perceiving a slightly different perspective of the same area. As we focus on objects closer to our eyes, our eyes rotate towards each other. As we focus on distant object, our eyes rotate towards a parallel view. The angle between the lines of sight of each eye is commonly termed the convergence angle. The convergence angle is higher when we view objects closer to our eyes and lower when viewing distance object. The convergence angle may be essentially zero, indicating essentially parallel lines of sight, when we view objects at great distance.
Three dimensional imaging, also known as stereographic imaging, dates at least as far back as 1838. Historically, stereographic cameras commonly include two lenses spaced laterally apart a similar distance as an average human's eyes, approximately 65 mm. The effective distance of the lenses from each other is known as the interocular distance. The interocular distance has a strong effect on the apparent depth of a stereographic image. Increasing the interocular spacing increases the apparent depth of a stereographic image. Decreasing the interocular spacing has the effect of decreasing the apparent depth of a stereographic image.
The presentation of stereoscopic images is commonly achieved by providing a first image to be seen only by the left eye and a second image to be seen only by the right eye. Differences, or disparity, between the two images may provide an illusion of depth. Two images having disparity may be perceived as three-dimensional. Two images, or portions of two images, exhibiting excessive disparity may not be perceived as three-dimensional, but may simply be seen as two overlapping two-dimensional images. A variety of techniques, including polarization, filters, glasses, projectors, and shutters have been used to restrict each eye to viewing only the appropriate image.
One approach to displaying stereographic images is to form the left-eye image on a viewing screen using light having a first polarization state and to form the right-eye image on the same viewing screen using light having a second polarization state orthogonal to the first polarization state. The images may then be viewed using glasses with polarizing lenses such that the left eye only receives light of the first polarization state and the right eye only receives light of the second polarization state. Stereoscopic displays of this type typically project the two polarized images onto a common projection screen. This technique has been used to present 3-D movies.
A second approach to displaying stereographic images is to form the left-eye and right-eye images alternately on a common viewing screen at a high rate. The images may then be viewed using shutter glasses that alternately occult either the right or left eye in synchronism with the alternating images.
Cinematic filming or recording is often done using fixed focal length lenses, commonly called prime lenses. For example, Carl Zeiss offers prime lenses in focal lengths of 3.9, 5, 7, 10, 14, 20, 28, 40, and 70 millimeters for use with ⅔-inch video cameras. With the exception of the longest and shortest focal length lenses, all of these lenses are physically the same size, and all of these lenses are compatible with the same matte box, filters, and motorized or manual control systems used to control focus and lens aperture. Thus a cinematographer or camera operator can change the field of view of a camera by exchanging one prime lenses for another. Similarly, Cooke Optics Limited offers prime lenses in 18 different focal lengths from 12 mm to 300 mm for use with 35 mm film cameras. Other manufacturers also offer prime lens sets for video or film cinematography.
Stereographic camera systems typically include two cameras having respective lenses. Stereography relies on differences between the images captured by the two cameras to create an impression of depth. However other factors, such as differences in the focal length, centration, or other characteristic of the lenses of the two cameras, may cause differences in the images captured by the two cameras that distort or obscure the desired impression of depth. Thus a stereographic camera system using prime lenses requires matched pairs of lenses for each focal length.
Throughout this description, elements appearing in schematic views and block diagrams are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits. Elements that have similar functions for either the left or right eyes are assigned the same reference designator with a suffix of either “L” or “R” to indicate left-eye or right-eye, respectively.